ERS-1 and JERS-1 are remote sensing satellites launched by the European Space Agency and NASDA of Japan, respectively. Both satellites carry Synthetic Aperture Radar instruments which are used to image the Earth's surface. The Alaska SAR Facility (ASF) has an agreement with both Space Agencies to receive data from these two sensors and process it into images. ERS-1 data can only be received within the viewing range of the ASF downlink antenna, which covers the state of Alaska, the Bering Strait, North-West Canada, the Gulf of Alaska, and the ice sheet up to the North Pole. JERS-1 data can be received by ASF for any part of the world. Data for each sensor can also be ordered direct from the European (ESRIN) or Japanese (NASDA EOC) data distribution facilities. Some restrictions and costs may apply.
The ERS-1 SAR is a C-band (6 cm wavelength) instrument, operating with VV polarization. It's orbit height is about 800 km and the range of incidence angles within a typical image is fixed at between 20 degrees for near range and 26 degrees for range range. A typical image area is 80 x 80 km.
The JERS-1 SAR is an L-band (24 cm wavelength) instrument, operating with HH polarization. It's orbit height is about 570 km and the range of incidence angles within a typical image is fixed at between 32 degrees at near range and 38 degrees at far range. A typical image area is 80 x 75 km.
The ERS-1 SAR was designed primarily to observe features on the ocean, such as waves or ice sheets. The wavelength and incidence angle selected for ERS-1 are also good for observing some, but not all, features on land, such as mountainous terrain or agricultural fields. The JERS-1 SAR was designed primarily to collect data over land which would be useful to geologists. The wavelength, incidence angle and polarization selected for JERS-1 are not ideal for observing ocean features.
ASF distributes hi-res (high resolution) and lo-res (low-resolution) data products. The hi-res data product has 8192 by 8192 12.5 x 12.5 m pixels, with 25-30m resolution and takes up 64 Megabytes of disk space. The lo-res ASF data product has 1024 by 1024 100 x 100 meter pixels, with 200 meter resolutionand takes up 1 Megabyte of disk space. Many samples (about 256) have been averaged to produce each pixel in the lo-res data products. Each pixel represents normalized radar cross section ([[sigma]]0) for either L-band HH or C-band VV at the appropriate incidence angle. The lo-res ASF products can be displayed by the MacSigma0 program.
The main difference between the lo-res ERS-1 and JERS-1 images versus those from an airborne system like the AIRSAR is the size of the imaged area, which is much larger for the satellite images. In addition, the resolution of the lo-res data is much larger than for the AIRSAR data. Also, the range of incidence angles for the satellite data is only about 6 degrees across the imaged swath, while for the AIRSAR the range of incidence angles is about 40 degrees for a typical image. This means that the backscatter variation with incidence angle across the swath is more pronounced for the AIRSAR data. The geometry for collecting satellite SAR data is similar to the geometry illustrated in the last section for the AIRSAR. The height of the platform, however, is significantly different. Since the AIRSAR platform (a DC-8 aircraft) flies at around 10 km altitude, to image a swath 10 km wide, an incidence angle variation of about 40 degrees is typical. For the satellite platforms, with altitudes greater than 500 km, an 80 km swath width can be observed with only a 6 degree variation in incidence angle across the swath.
For more information on the Alaska SAR Facility processor
"User's Guide To Products", Jet Propulsion Laboratory Document D-9362, Version 1.0, Jet Propulsion Laboratory, Pasadena, California, January 1992.
An ERS-1 or JERS-1 data set processed at the Alaska SAR Facility contains three files: the data file itself, a trailer file, and a leader file. The data file contains a small header and the data itself. In the data, there is generally some number of bytes at the beginning of each line which are ignored. This number is typically 12. The following is a typical data file:
The leader and trailer files are both of the same format. This format is called the CEOS format. In this format, files contain a sequence of "records" which are saved end-to-end. Each "record" is a particular type. Encoded in the beginning of each record are the record type and record length. The parameters contained within each "record" and their specific locations depend on the record type. Unfortunately, the encoded record type is not consistent. The best method of locating records is by using the File Descriptor Record which is always the first record in either file. This record contains information about the other records contained within the file. Also, the sequence of "records" is sometimes not actually stored end-to-end but may be separated by an extra byte. The MacSigma0 program was written to check and compensate for this extra byte.
The leader and trailer files are identified as ".ldr" (or ".LDR") and ".trl" (or ".TRL", ".tlr", or ".TLR") files. In other words, if the data file is called "Beaufort", then the trailer file should be either "Beaufort.trl" or "Beaufort.TRL". If the data file is called "Bonanza Creek.dat", then the trailer file is expected to be either "Bonanza Creek.trl", "Bonanza Creek.TRL", "Bonanza Creek.tlr", or "Bonanza Creek.TLR".
The following are references for the CEOS header format:
"Alaska SAR Facility (ASF): System Interface Specification", Jet Propulsion Laboratory Document D-5267 Revision A, Jet Propulsion Laboratory, Pasadena, California, May 1992.
SAR Data Products Standard
Committee on Earth Observing Satellites
Working Group on Data
Revision 2.0, 10 March 1989
(available from the user services group at the Alaska SAR Facility at 907-474-7487)